Challenge: MythBusters and Cruisers Forum

[ThinAirDesigns]

Hi there. A while back there was a discussion involving a turbine driven cat that morphed into a discussion of the associated physics here on CF.

http://www.cruisersforum.com/forums/...-cat-9690.html

A buddy and I have entered the following related video into the MythBusters video challenge:

YouTube - Directly Downwind Faster than the Wind (improved audio)

Since this site is one of those referred to in our video I though it might prove interesting to y'all. Take it or leave it.

Thanks.

JB

[BigCountyEsq]

very interesting. Please keep us informed of your progress.

[GDFL]

I'm impressed. Good luck

[Stillraining]

Cool!!

[David M]

Not only that, it would reduce the seagull population!

[scotty]

i personally dont think a prop driven machine can go faster than the wind, at some point the wind will be from the front, ie the relative wind. now if the gear box allowed the prop to continue spinning while shift directions of the output it would then be drivin by the relative wind. the tread mill unit is using a tread mill as the power source to drive a prop pushing it forward as long as the pitch of the prop is faster than the tread mill speed it will go faster than the tread mill. until the gear ratio evens out, ie prop pitch speed works out to 15 mph and the tread mill speed is 10, the machine will reach 15, if you bumped the tread mill up 10 % the prop speed will go up 10 % times the gear ratio. the tread mill machine is limited by weight and friction, too much friction and it will loose speed, too much weight ( thrust needed ) it will need a helping hand to get up to speed before the prop can push it

[scotty]

sorry second post, the big difference is the tread mill is providing the energy, its just getting pushed thru the wheels to the prop, the machine only needs enough thrust and prop speed to over come the friction on the drive train, the relative speed is small ground speed is higher.

to take it to the famous can a plane take off on a tread mill, yes it can, ground speed is higher but airspeed stays the same. that same plane could not take off if its brakes where locked

[perchance]

When they start handing our free lunches sign me up.

The plane on a treadmill was done on Mythbusters a while ago. It seemed simple to me, but I look at things differently. It is the prop through the air, not the wheels across the ground that bring a plane up to a speed where the air is passing over the wings at a speed that will create sufficient low pressure to lift the plane.
So, in simple terms, this is a very different challenge. There is no question in my mind this will work, as it is a simple matter of power conversion. Wind driving a prop, to wheels driving an object. For the application of a wheeled vehicle, there are less resistances to overcome than there are on a water craft. I do not believe that a wind turbine watercraft could produce enough torque to exceed wind speed.

[Robertcateran]

WHen the wind is 15knots and the boat is doing 15 through the water, then there is no wind on the turbine and therefore no power to the prop

[perchance]

What about all these wind turbines.If there is forward thrust wouldn't that put excessive strain on the towers and the bearings on the generators? It seems to defy logic but what do I know, I'm just a grunt.

[David M]

If you think about it, sails and windmills are both foils propelling a boat. The disadvantage of the windmill is you have the inefficiency of converting energy produced by a foil to mechanical energy and back to energy produced by a foil at the prop.

Given mechanical losses, a windmill cant possibly be more efficient. With sails there is no loss to converting energy, twice.

I don't see how there could be a propulsion advantage with respect to efficiency of a windmill powered boat.

[cal40john]

You guys are a few thousand posts behind and doing the same arguments I've seen on the less polite forum and few changed sides. Perhaps the solution is to spend the $20 on parts and try it. I don't claim to know, and won't insist one way or another until I do it for myself, but a couple of interesting thoughts I've seen include:

1. The force generated by foils is related to apparent wind speed. What apparent wind does the foil see on a rotating (moving) mechanism on your craft?

2. An ice boat's vmg DDW is reported as being faster than true wind by reaching and jibing. Tie a string from the ice boat to a sled and drive the sled DDW, call the whole thing one craft.

John

A propellor through air works differently than a propellor through water. That is why a wind turbine can drive a vehicle across land faster than actual air speed. While it is true apparent wind speed will reach zero when actual vehicle speed matches actual wind speed, a propellor, like a sail or like a wind, works not only by cutting through the wind like a propellor through water, but, like a sail or wing, but creating a low pressure area in front of the blade. This low pressure area creates a vacume that pulls the vehicle forward, and the pocket created will allow the propellor to continue to propell the vehicle forward even after apparent wind speed hits zero. At least, that is the theory as I understand it. In practice, there are other variables, and the efficiency of the propellor is the main one. If the efficiency of the prop can not produce sufficent vacuum to keep up with that vacuum pocket, it can not reach, or exceed windspeed. Because the efficiency of the prop is so crucial, and prop design is such an exact science, it is not likely that most of us here would be able to put a test vehicle together from parts found at the local hardware store.

[Robertcateran]

An ice boat can et down wind faster than windspeed by using the energy generated from reaching to go faster than the wind, then using the momentum to go down wind faster than the wind. It then experiences a head wind. We can all tack into the wind. Is it theoretically possible with efficient foils to keep going faster and faster? Does this mean that with efficient foils, on a still day, get a shove and then continually accelerate? I strongly suspect there is some perpetual motion problem here. I'd love it if I was wrong. I do know that at windspeed it has zero power input but it my be possible to go through this deadspot if there is a sufficiently efficient flywheel to help it through this powerless area